The Human Genome Project was funded and pursued based on the premise that the sequencing of the human genome would reveal the genetic basis for complex diseases that have a strong inheritable component, including cardiovascular disease, neurodegenerative conditions, and metabolic diseases such as diabetes. It was believed that this information would lead to new drug targets for these widespread diseases. However, thousands of genome-wide association studies (GWAS) have shown that the genetic variation associated with these complex diseases does not occur within genes, but rather in intergenic regulatory regions that control the levels of particular genes. Similarly, approximately 20% of Mendelian disorders do not have a detectable coding mutation, suggesting that the causal mutation is in a gene regulatory element. Importantly, it is very difficult to assign functional roles to these regulatory elements as they often are located in distant locations from their target genes. Moreover, many genes and regulatory elements fall into each positive hit of each GWAS study. In fact, follow-up projects to the Human Genome Project, such as the NIH-funded Encyclopedia of DNA Elements (ENCODE) and the Roadmap Epigenomics Project, have identified millions of putative regulatory elements across the human genome for many human cell types and tissues.
A primary challenge of functional genomics is to develop technologies that directly and precisely manipulate genome function at individual loci. Projects such as ENCODE and the Roadmap Epigenomics Project have identified millions of epigenetic marks across the human genome for many human cell types and tissues. Studying the function of those marks, however, has been largely limited to statistical associations with gene expression. Technologies for targeted direct manipulation of these epigenetic properties are necessary to transform such association-based findings into mechanistic principles of gene regulation. Such advances have the potential to benefit human health, as they could lead to gene therapies that modify the epigenetic code at targeted regions of the genome, strategies for regenerative medicine and disease modeling based on the epigenetic reprogramming of cell lineage specification, and the engineering of epigenome-specific drug screening platforms.
Manipulation of the epigenome is possible by treating cells with small molecule drugs, such as inhibitors of histone deacetylases or DNA methyltransferases, or differentiating cells into specific lineages. However, small molecule-based methods globally alter the epigenome and transcriptome, and are not suitable for targeting individual loci. Epigenome editing technologies, including the fusion of epigenome-modifying enzymes to programmable DNA-binding proteins such as zinc finger proteins and transcription activator-like effectors (TALEs), have been effective at achieving targeted DNA methylation, DNA hydroxymethylation, and histone demethylation, methylation, and deacetylation.
Fused to activation domains, such as oligomers of the herpes simplex viral protein 16 (VP16), dCas9 can function as a synthetic transcriptional regulator. However, limitations in the use of dCas9 activators remain, including the need for multiple activation domains or combinations of gRNAs to achieve high levels of gene induction by synergistic effects between activation domains. The conventional activator domains used in these engineered transcriptional factors, such as the VP16 tetramer VP64, function as a scaffold for recruiting multiple components of the preinitiation complex and do not have direct enzymatic function to specifically modulate the chromatin state. This indirect method of epigenetic remodeling does not allow for testing the role of specific epigenetic marks and may not be as potent as the direct programming of epigenetic states. There remains a need for the ability to target direct manipulation of epigenetic properties.